Fuel, lubrication oil, hydraulic oil and the like have been widely used in various machines. For example, a construction machine, such as a hydraulic excavator, uses a fuel to work an internal combustion engine for driving a hydraulic pump, a lubrication oil to lubricate the internal combustion engine and the like, and a hydraulic oil to transmit hydraulic pressure from the hydraulic pump for the purpose of running the construction machine and driving its working mechanism. Such fuel, lubrication oil, hydraulic oil and the like are stored in respective tanks. If the construction machine is not operated over a long period, the fuel, stored in the tank and left still without addition of any fresh fuel, is gradually deteriorated. The lubrication oil and the hydraulic oil are apt to deteriorate by not being refilled or renewed for long periods of time. All these three can be deteriorated if added or mixed with an inferior fuel or oil. Such deterioration causes undesired matters like performance degradation of the internal combustion engine and a decline in operation efficiency or durability of the construction machine. Thus, earlier and timely renewal of the fuel and oils is required before the deterioration progresses.
In late years, biofuels, derived from biomass for example, have been increasingly used as alternative fuels for the internal combustion engine. Biofuel is advantageous in that greenhouse gas emission can be reduced but, on the other hand, is disadvantageous in that metal parts of a fuel supply system are corroded as the biofuel is oxidized, and that proper fuel injection control becomes difficult to perform as the viscosity of the biofuel increases due to oxidation. For this reason, taking care of the deterioration of biofuels is of great importance when the biofuels are used as fuels for the internal combustion engine.
Various measures have been suggested to deal with the above problems. Technologies described in Patent Documents 1 and 2 can be cited as examples of a technology to detect a quality deterioration of biofuels. Based upon the phenomenon in which the color shade of a biofuel changes with the progress of oxidation of the biofuel, the technology disclosed in the Patent Document 1 periodically detects the color shade in terms of light transmittance, sequentially calculates the amount of change with respect to a previous value, and determines a degree of deterioration of the biofuel on the basis of an integrated value obtained by accumulating absolute values of the amounts of change.
According to the technology disclosed in the Patent Document 2, based upon the phenomenon in which the oxygen concentration of a biofuel increases and delay in ignition shortens as the oxidation of the biofuel progresses, an ignition delay derived on the basis of the in-cylinder pressure during operation of the internal combustion engine. If the derived ignition delay is shorter than a reference value by a predetermined amount or more, it is determined that the biofuel is deteriorated.